While it is dystopian that many people have to end up working beyond retirement age, I can see why some people choose to take on a "retirement job".
Our jobs are what we do for the majority of our time, for better or worse, so once you retire there's suddenly a purposeless void where work used to be. To keep your mind going and your body moving, you need to fill that void with things you want to do - but for a lot of people it's hard to do that sustainably, so after a few years they need something else, and for some people that's a "retirement job".